<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
      list-style: none;
    }

    html,
    body {
      width: 100%;
      height: 100%;
    }

    /* → 将Flex布局, 设置到 "父元素" 上, 这样 子元素
      才会实现 flex-水平垂直居中 的效果..... */
    .reg-container {
      display: flex;
      justify-content: center;
      align-items: center;
      width: 100%;
      height: 100%;
      background-color: skyblue;
    }

    .reg-box {
      width: 450px;
      height: 300px;
      background-color: pink;
    }
  </style>
</head>
<body>
  <div class="reg-container">
    <div class="reg-box"></div>
  </div>
</body>
</html>